In 2009, there was a call for ambitious proposals to use Hubble for projects that were beyond the scope of what a typical time allocation could accomplish. Hubble time is usually doled out in “orbits.” One orbit of Hubble takes about 90 minutes yielding 45 minutes to an hour of observing time (because the Earth typically blocks a portion of the sky from view). A typical proposal will be for a few orbits of observing time. In this particular call, proposers were asked to consider projects needing at least 450 orbits.

Two teams responded to this call with very ambitious proposals to observe representative patches of sky to search for the most distant galaxies, study the assembly of galaxies over cosmic time, trace the formation of black holes in the centers of galaxies, and study distant supernovae. The proposals were similar in many respects, and the time allocation committee recommended merging the two teams. Thus the CANDELS collaboration was formed, with participation of nearly 100 astronomers with diverse backgrounds and interest. The time allocation was 902 orbits, which is the largest in the history of the Hubble telescope.

Why did so many astronomers – on the proposal teams and the time allocation committee – think this kind of observation was important? And what have the observations revealed?

The answer to the first question goes back to a fundamental assumption of cosmology – that the universe is basically the same in all directions. Obviously this assumption breaks down on small scales (otherwise there wouldn’t be planets, stars, and galaxies), but it appears generally true when averaging over scales larger than about 10 million light years. The Hubble observations allow us measure the past: to observe galaxies and supernovae that are so distant that their light has taken billions of years to reach us. Any single Hubble image will have both nearby galaxies and galaxies for which the light-travel time more than 13 billion years (the universe itself is 13.8 billion years old). To get a reasonably fair census of the distant universe, we need to point at places that are out of the plane of the Milky Way galaxy. We need to take fairly long exposures to collect enough photons. We should observe these same patches at other wavelengths (from x-ray to radio). All else being equal, we should divide the total area into several patches that are disjoint on the sky to reduce systematic errors due to foreground dust or large-scale cosmic structures. Hence the CANDELS survey: a public Hubble survey of the most-studied patches of sky, coordinated with observations from other major observatories.

The CANDELS observations were completed in 2013 and so far there have been over 200 papers published using the data. It’s possible to give only at taste of the scientific results in this blog article. There are many more summaries on CANDELS blog site.

Cosmic Dawn

Ever since the installation of the WFC3 camera on Hubble in 2009, the race has been on to identify the most distant galaxies. It was unclear at the outset which strategy would be most successful: taking very deep exposures over a tiny area, shorter exposures over a wider area, or pointing at galaxy clusters and using gravitational lensing to magnify galaxies in the background. Over the course of several years, Hubble has done all three, and the current record holders are in one of the CANDELS fields and in the background of a cluster of galaxies. Follow-up observations of a bright candidate in the CANDELS GOODS-North field suggest that it is at a redshift z=11.1, about 400 million years after the Big Bang (Oesch et al. 2016).

Aside from the lure of seeing the most distant galaxies, there is much to learn from studying statistical properties enabled by the large survey – with samples now approaching 1000 galaxies within the first billion years and 10000 within the first two billion. (Prior to the installation of WFC3 and the CANDELS survey, there were only a handful of good candidates identified at these early times.) There appear to be enough of these very young galaxies to explain the rather rapid “re-ionization” of the universe. About one billion years after the big bang there was a huge injection of energy that stripped 99.99% of the electrons away from the protons in the hydrogen between galaxies. The observations show that there was enough energy in young galaxies to explain this; although we are not yet certain that enough of the photons at just the right energy to ionize hydrogen can escape, because the gas within the individual galaxies might absorb most of it. Galaxies in the first billion years have bluer colors than their counterparts at later epochs – probably because they have not yet had enough time to build up the heavy elements needed to form large amounts of dust and to lower the temperatures of young stars. Nevertheless, in spite of being bluer, few if any of the galaxies show the very blue signature expected of galaxies forming their first generation of stars. Comparing the evolving numbers and stellar masses of galaxies to the theoretically-predicted numbers of gravitationally-bound dark-matter “halos,” leads to the conclusion that the star-formation rates are almost – but not entirely – governed by the somewhat clumpy inflow of gas as the gravitational pull of the newly formed dark-matter halos draws in more gas from the surrounding intergalactic medium.

Figure 1: The left panel shows the number of very distant galaxies identified by the CANDELS survey (red) and deeper surveys (blue) since the WFC3 camera was installed on Hubble. The right panel shows the estimate of the “cosmic star-formation rate” – the number of stars formed per year in a fixed volume of the universe – as a function of time since the Big Bang.

The addition of infrared wavelengths – both from Hubble and from the Spitzer and Herschel observatories at longer wavelengths – has been essential for searching for galaxies that are either full of dust or shutting off their star formation. Such galaxies are red enough that they are difficult to pinpoint as distant-galaxy candidates in the Hubble images alone or entirely invisible in the Hubble images. Massive dusty or “quenched” galaxies are expected to be extremely rare in the early universe because there simply hasn’t been time for them to form. Nevertheless, there are dozens of interesting candidates found in the CANDELS fields when inspecting the infrared images. These will high-priority targets for spectroscopy with JWST and ALMA, which will be able to confirm their distances.

Cosmic High Noon

The overall cosmic rate of star formation peaked at a redshift z ≈ 2, when the universe was about 3-4 billion years old. The CANDELS observations provided the first large samples of galaxies with high-resolution images spanning wavelengths from the rest-frame ultraviolet to the optical. The longer wavelength data from Spitzer helps to pin down the total stellar masses of the galaxies, by providing extra sensitivity to some of the oldest, reddest stars. Using samples of tens of thousands of galaxies, we are able to assess the successes and failures of our current theoretical understanding of galaxy evolution, and provide some clues to guide future developments. The observations tell us that something is “quenching” the star-formation in massive galaxies as early as 2-3 billion years after the Big Bang. These quenched galaxies emerge as very compact “red nuggets,” which must grow substantially in size and over the next ten or so billion years, increasing in mass mostly by merging with neighboring galaxies rather than forming new stars in situ. The compact star-forming progenitors of these galaxies (blue nuggets) appear to be present in sufficient numbers to account for the red nuggets, but we do not yet entirely know how or why star-formation is shutting down. The blue nuggets have a somewhat higher incidence of active nuclei: central black holes that are accreting gas at a high rate, and perhaps heating the gas that would otherwise cool to form stars. Quenched galaxies have higher central densities of stars than most star-forming galaxies, so the thought is that when sufficiently large amounts of gas collect in the center, this triggers a burst of star formation and perhaps also feeds an active nucleus. The energy feedback from the star formation and the nucleus are sufficient to shut off subsequent star formation. High-resolution computer simulations of forming galaxies suggest that the trigger for this gas funneling is a mix of gravitational instabilities within a star-forming disk of gas and mergers with surrounding galaxies. When dust is included in these simulations, they look remarkably like the galaxies we see, but differ enough in their statistical properties (for example their colors) that we know that some aspects of the physical models are not quite correct.

Figure 2: Computer simulations vs. observations. The bottom panels show some of the highest-resolution hydrodynamical simulations of galaxies that have yet been constructed on supercomputers. The images in the middle show the same galaxies viewed from two different camera directions and placed at a large distance from the telescope so that our view matches what we might see from Hubble. The top panels show galaxies selected from the CANDELS survey. Qualitatively, the computer simulations doing a very good job of matching what we see in deep observations.

Towards the present day

CANDELS has provided us with large enough samples of galaxies that it is possible to try to find examples of what the Milky Way galaxy might have looked like in the past. We can attempt to match progenitors to descendants in the overall population of galaxies by isolating galaxies that are at about the same rank in the overall ranking of galaxies by stellar mass (from biggest to smallest). Figure 3 shows a visual summary of the results of this kind of effort – in what might be considered to be a family tree of the Milky Way. The progenitors are smaller, bluer, and generally do not have the familiar spiral-plus-bulge structure that we see in present-day galaxies. The same study provides a way to infer the amount of cold gas that ought to be present as fuel for star formation, and these predictions are being tested with ongoing observations from the ALMA observatory.

Figure 3: Examples of progenitors of a Milky-Way-mass galaxy taken from the CANDELS survey. Redshift and time (in billions of years since the big bang) run along the horizontal axis. The figure has been divided into three panels for convenience; the earliest times are at the bottom and the latest times are at the top. The galaxies are shown to the same physical scale and the colors are a fair representation of their rest-frame colors. The position along the vertical direction illustrates how blue (or equivalently, hot) the galaxy is, with red toward the top and blue toward the bottom.

A main caveat in current statistical studies of galaxies at z ~ 1 is that the way in which the physical properties of galaxies, such as stellar mass (M∗) and star formation rate (SFR), are generally derived from multi-wavelength datasets does not reflect recent advances in the modeling of galaxy spectral energy distributions (SEDs). For example, spectral analyses often rely on oversimplified modeling of the stellar spectral continuum using simple star formation histories (SFHs), such as exponentially declining τ-models. Some studies have shown that more sophisticated SFH parametrizations provide better agreement with the data (e.g. Lee et al. 2010, 2014; Pacifici et al. 2013; Behroozi et al. 2013). The inclusion of nebular emission is also important to interpret observed SEDs of galaxies. Elaborate prescriptions have been proposed, based on combinations of stellar population synthesis and photoionization codes. We have investigated, in a systematic way, how different SED modeling approaches affect the constraints derived on the physical parameters of high-redshift galaxies.

We used version 4.1 of the 3D-HST Survey photometric catalogue for the GOODS-South field covering an area of 171 arcmin2 (Skelton et al. , 2014). We compiled a sample of 1048 galaxies at redshifts 0.7 < z < 2.8 (H < 23) with accurate photometry at rest-frame UV to near-IR wavelengths (U, ACS-F435W, ACS-F606W, ACS-F775W, ACS-F850lp, WFC3-F125W, WFC3-F140W, WFC3-F160W and IRAC 3.6μm). Grism (low-resolution) observations provided us with reliable spectroscopic redshifts (Brammer et al. 2012) for all galaxies and good optical emission-line equivalent widths (EW) for a subsample of 364 galaxies.

We considered three modeling approaches relying on different assumptions: the explored (prior) ranges of star formation and chemical enrichment histories; attenuation by dust; and nebular emission. We built:

The same P12nEL spectral library, only including also the nebular component (P12, Pacifici et al. 2012).

In Figure 1, we compare the observer-frame colors of the galaxies in the sample (grey symbols) with the predictions of the three model spectral libraries (colored contours). This figure shows that the CLSC spectral library leaves few observed galaxies with no model counterpart. Thus, SED fits for these galaxies will be biased towards the models that lie at the edge of the spectral library. The P12nEL spectral library can cover reasonably well the bulk of the observations, showing the importance of accounting for more realistic ranges of SFHs and dust properties than included in the CLSC spectral library. Few observed galaxies fall outside the contours of the P12nEL model spectral library, presumably because of the contamination of the WFC3-F160W flux by strong Hα emission. The P12 spectral library allows us to cover reasonably well the entire observed color-color space.

Figure 1: Optical-NIR color-color diagrams comparing the 3D-HST sample (grey symbols; open circles mark objects for which error bars are larger than 0.2 mag) with the three model libraries as labeled on top (contours; the three lines mark 50, 16 and 2 per cent of the maximum density).

We compared the constraints on M∗ and SFR derived for all 1048 galaxies in the sample using the CLSC and P12nEL model spectral libraries to those obtained using the more comprehensive P12 library. We summarize the results in Table 1. The use of simple exponentially declining SFHs (CLSC spectral library) can cause strong biases on both the M∗ (~ 0.1 dex) and the SFR (~ −0.6 dex). Not including the emission lines in the broad-band fluxes (P12nEL) does not strongly affect the estimates of M∗, but can induce an overestimation of the SFR (~ 0.1 dex).

Table 1: 16, 50 and 84 percentiles of the distributions of the differences between best estimates of M∗ and SFR when comparing the constraints obtained with different libraries.

To further quantify how emission lines contaminate observed broad-band fluxes, we recorded, for a subsample of the 3D-HST galaxies (364), the contribution by nebular emission to the WFC3-F140W magnitude of the best-fitting P12 model (as derived when including the constraints from both 9-band photometry and EW measurements from 3D-HST grism data). This is shown in Figure 2 as a function of stellar mass. For galaxies at redshifts 0.8 < z < 1.4, both Hα and [S II] fall in the WFC3-F140W filter. Crosses represent the combined contamination caused by these lines. In the same way, we plotted the combined contamination by Hβ and [OIII] for galaxies in the range 1.5 < z < 2.2 (empty circles). Each galaxy is color-coded according to star formation activity, from low (red) to high (black) specific SFR. For galaxies at 0.8 < z < 1.4, the contamination decreases from ~ 0.1 to ~ 0.02 mag as the stellar mass increases from ~ 109.5 to ~ 1011 solar masses. At higher redshift, where [O III] and Hβ are sampled in the band, the contamination is slightly larger because the SFR is on average larger at higher than at lower redshifts (Noeske et al. 2007).

Figure 2: Contribution by nebular emission to the WFC3-F140W magnitude of the best-fitting (P12) model for a subsample of 3D-HST galaxies (for which good grism observations are available) as a function of stellar mass. Crosses represent galaxies with measured Hα and [S II] (0.8 < z < 1.4), while circles represent galaxies with measured Hβ and [O III] (1.5 < z < 2.2). The points are color-coded according to star formation activity from low (red) to high (black) specific SFR. The contamination of the emission lines in the broad-band WFC3-F140W filter increases from ~ 0.01 to ~ 0.5 mag as the stellar mass decreases.

The results obtained in this work revealed the importance of choosing appropriate spectral models to interpret deep galaxy observations. In particular, the biases introduced by the use of classical spectral libraries to derive estimates of M∗ and SFR can significantly affect the interpretation of standard diagnostic diagrams of galaxy evolution, such as the galaxy stellar-mass function and the main sequence of star-forming galaxies. In this context, the spectral library developed by Pacifici et al. (2012) offers the possibility to interpret these and other fundamental diagnostics on the basis of more realistic, and at the same time more versatile models. This is all the more valuable in that the approach can be straightforwardly tailored to the analysis of any combination of photometric and spectroscopic observations of galaxies at any redshift.

One of the most important problems in modern astrophysics is to understand the co-evolution of galaxies and their central supermassive black holes (SMBH) (see e.g. Heckman & Best 2014 for a recent review). Since the matter that ultimately accretes onto the central black hole needs to lose almost all (~99.9%) of its angular momentum, studies of mergers, tidal interactions, stellar bars and disk instabilities are central for understanding the details of such a process. However, observational efforts to assess the importance of mergers in Active Galactic Nuclei (AGN) so far have led to conflicting results. A major issue is related to the so-called radio-loud/radio-quiet dichotomy of active nuclei. Radio-loud AGNs have powerful relativistic plasma jets that are launched from a region very close to the central SMBH. The most popular scenario among those proposed so far assumes that energy may be extracted from the black hole via the innermost region of a magnetized accretion disk around a rapidly spinning black hole Blandford & Znajek (1977). In such a framework, the radio-quiet/radio-loud dichotomy can be explained in terms of a corresponding low/high black hole spin separation (Blandford et al. 1990). It is also important to stress that radio-loud AGN are invariably associated with central black holes of masses larger than ~108 solar masses (e.g. Chiaberge & Marconi 2011). Therefore, the black hole mass must play a role.

With the aim of determining the importance of mergers in triggering different types of AGN activity, my collaborators and I selected 6 samples of both radio-loud (RLAGN) and radio-quiet (RQAGN) AGN, and of non-active galaxies matched to the AGN samples in magnitude (or stellar mass). We focused in particular on redshifts between z=1 and z=2.5. All objects were observed with HST/WFC3-IR at 1.4 or 1.6mm, in order to ensure appropriate sensitivity at rest-frame optical wavelengths, and to allow us to detect faint signatures of a merger event. Most of the objects were taken from large surveys performed with Hubble (CANDELS, 3D-HST). The images of the high-luminosity radio galaxies were taken from a “snapshot” program we performed as part of our 3CR-HST survey of radio-loud AGN.

The fun part of the work was to visually inspect the WFC3-IR image of each of the 168 objects (Fig. 1) and determine whether each object was or was not showing signatures of a merger, according to a pre-defined classification scheme. It was at that point that we found something really interesting. Without knowing what type of object we were looking at, we classified almost all (95%) of the RLAGNs as “mergers”. On the other hand, the RQAGN samples and the non-active galaxies had merger fractions between 20% and 37%. We performed a careful statistical analysis of the results, and we concluded that the merger fraction in RLAGN is significantly higher than that in RQAGNs and non-active galaxies (Fig. 2). It is possible that all RLAGNs are associated with mergers. This result was also confirmed for lower redshift samples of radio galaxies (one at z~0.5 and one at z<0.3), and for objects of both low and high power. On the other hand, the merger fraction of RQAGNs is statistically not different from that of non-active galaxies.

Figure 2 Merger fraction vs. average radio loudness parameter Rx (ratio of the radio to X-ray luminosity) for the different AGN samples. Radio-quiet AGNs are on the left of the dashed line, radio-loud AGN are on the right. The filled symbols are the radio-loud samples and the empty symbols are radio-quiet. The dashed line represents the radio loudness threshold for PG QSOs. The solid line marks the 60% merger fraction that appears to roughly separate radio-loud and radio-quiet samples.

This result has very important implications. Firstly, it shows a clear association between mergers and AGN with relativistic jets (the RLAGN subclass), with no dependence on either redshift or luminosity. Secondly, we firmly determined that not all AGNs are triggered by mergers. The question now is how do mergers trigger AGN with jets? A possible scenario we envisage is that when a galaxy merger happens, the central supermassive black holes merge as well. In general, the resulting spin of the BH after coalescence is lower than the original spin values. But for particular spin alignments and for BHs of similar masses, the spin can be significantly higher (see Schnittman 2013, for a recent review). In that case, if the mass of the BH is at least ~108 solar masses, the energy extracted through the Blandford-Znejek mechanism may be large enough to power the jet. This is not a completely new idea, since it was already proposed in a slightly different form by Wilson & Colbert (1995). In the near future, we will focus on confirming the strong connection between RLAGNs and mergers with a larger dataset of HST observations, ALMA observations, and integral-field spectroscopy.

Almost 25 years of HST observations have shown that most, if not all, large galaxies harbor a super-massive black hole (SMBH) in their nucleus. How and when these exotic objects formed is one of the puzzles of current astronomy. Many black hole studies are focused on images of the high-redshift universe because those can unveil the structure of galaxies in their distant past, and thus tackle the “chicken-and-egg” question of what came first, the black hole or the galaxy.

An alternative path to addressing this question is to study low-mass nearby galaxies, which have only very small central black holes or none at all. Why are these black holes “lagging” in their evolution? Are they still growing, and if so, what regulates their growth?

The answers most likely have to consider the presence of another type of compact massive object often found in galactic nuclei, namely an extremely massive and dense cluster of stars. In fact, these so-called “nuclear star clusters” (NSCs) are the densest stellar systems known, with many millions of stars packed in a radius of only a few light years. To measure their compact structure requires the highest possible spatial resolution, and consequently, NSCs have been studied systematically only during the last decade or so.

It has become clear that NSCs are found in nearly all low-mass galaxies, and that they are an essential ingredient for any recipe to understand the evolution of galactic nuclei. They are most easily observed in the absence of a luminous bulge, which is why our work is focused on late-type spiral galaxies. Fig. 1 shows a prototypical example of a NSC, namely the one in the nearby bulge-less spiral NGC 1042. This NSC is known to contain a low-luminosity SMBH with a mass of less than a million solar masses.

Other well-known examples for a SMBH within a NSC are NGC 4395, and of course our own Milky Way. On the other hand, the Triangulum Galaxy (Messier 33) also has a NSC and is very similar to NGC 1042 in mass and size, but it does not appear to contain any SMBH in its nucleus, at least none more massive than a few tens of thousands solar masses.

Why then do SMBHs exist within some NSCs, but not in others? What regulates the relative importance of the two types of central massive objects, i.e. the ratio of their masses? Does a SMBH destroy its parent cluster once it reaches a certain mass? Or does the presence of a NSC prevent the SMBH from growing its mass any further?

A systematic comparison of NSCs with and without confirmed SMBHs promises to shed light on these questions. Unfortunately, at present there are only a handful of galaxies known to host both a NSC and a SMBH, thus making a statistically sound comparison difficult. In order to improve this situation, my collaborators and I are working to develop observational methods to find more systems with coexisting NSC and SMBH. The challenge here lies in the fact that in this mass range, SMBHs are very difficult to detect, because they are much less active than their more massive counterparts.

We begin by using HST imaging to constrain the luminosities, sizes, and masses of NSCs in a large number of nearby spiral galaxies. We have recently completed a systematic analysis of all WFPC2 images in the HST archive that contain NSCs. By carefully analyzing the color and shape of the NSCs, we find some cases with point-like residual emission which may be indicative of an active galactic nucleus, and thus of a SMBH. Such residuals are, in fact, evident in NGC 1042 (see the inlays in Figure 1) – they likely are caused by the “extra” emission from the SMBH.

We then follow up these SMBH candidates with adaptive optics-assisted ground-based spectroscopy that will allow us to measure the age and total mass of the NSC, and to search for spectroscopic signs for the presence of a SMBH. On the theoretical front, we try to improve our understanding of dense stellar systems by analyzing poorly understood effects caused by the presence of large amounts of gas in the early days of NSC formation which may lead to an evolving stellar mass function via gas accretion, or to runaway growth of stellar-mass black holes.

In this way, we hope to better understand the mutual interaction of SMBHs and NSCs, and to ultimately to learn how “monster” black holes in massive galaxies have formed.

Stellar halos are built largely from the accretion and disruption of satellite galaxies. Tidal debris features from these disruption events remain identifiable for billions of years, providing observable signatures of the merger histories of individual galaxies. However, in situ star formation may also play a part in building up the inner regions of stellar halos, either via stars formed in the proto-disk of the host galaxy, or in gas deposited by disrupted satellites.

Therefore, studying stellar halos in detail provides a window into the formation histories of galaxies. With current instrumentation, the only stellar halos that can be studied in great detail are the Milky Way and Andromeda (aka M31), the two large spiral galaxies of the Local Group.

I am a member of the SPLASH collaboration (Spectroscopic and Photometric Landscape of Andromeda’s Stellar Halo). Our team has amassed a large photometric and spectroscopic dataset of red giant branch stars in M31′s halo and dwarf galaxies. Our photometric data are primarily taken with the Mosaic camera on the Mayall 4-m telescope on Kitt Peak, and include narrow band imaging that allows us to select spectroscopic targets with a high probability of being M31 stars. Our spectroscopic data are taken with the DEIMOS multi-object spectrograph on the Keck II 10-m telescope.

Figure 1: The locations of our Keck/DEIMOS spectroscopic fields in M31′s stellar halo, overlaid on the PAndAS starcount map (McConnachie et al. 2009). Our spectroscopic observations target fields on and off halo substructure, and cover a large range in radius.

My recent focus is on leveraging the full M31 halo dataset (shown in Figure 1) to learn about the global properties of M31′s stellar halo, and the ensemble of disrupted dwarf galaxies that built it. We have used this dataset to show that M31′s stellar halo extends to at least 180 kpc in projection from the center of M31. Furthermore, the density profile of stars shows no indication of a break, even though we are now tracing it to 2/3 of M31′s virial radius (Gilbert et al. 2012).

I am currently using the M31 halo dataset to analyze the metallicity distribution of M31 halo stars as a function of radius. We have spectra of over 1500 M31 stars in 32 spectroscopic fields ranging in distance from 9 to 180 kpc from M31′s center. The data show a clear gradient in metallicity that extends to 100 kpc (Figure 2). This gradient is seen in both the photometric (based on a star’s position in the color-magnitude diagram) and spectroscopic (based on the strength of the Calcium II triplet absorption feature) metallicity estimates.

Our spectra allow us to analyze the velocity distributions of stars in each field and to identify tidal debris features by their cold kinematical signatures. When we remove stars associated with tidal debris, the strength of the observed metallicity gradient increases.

Figure 2: Metallicity Distribution Functions of stars in M31′s halo: (top left) all M31 halo stars; (top right) after removal of tidal debris features; (bottom) cumulative distributions for all halo stars (solid curves) and after removal of tidal debris features (dashed curves). Arrows mark the median [Fe/H] values for each distribution. The inner halo is primarily metal-rich, while the outer halo is significantly more metal-poor. The data show evidence of a metallicity gradient in M31′s stellar halo extending from 9 kpc to ~100 kpc.

This large-scale metallicity gradient, when compared to the results of simulations of stellar halo formation, implies that the bulk of M31′s stellar halo was likely built primarily from one to a few relatively massive dwarf galaxies (>109 solar masses).

However, we also observe significant field-to-field scatter in the mean metallicities and surface brightnesses of fields at large radius. This implies that recently accreted, small dwarf galaxies have contributed substantially to the outermost regions of M31′s stellar halo.

If you are interested in learning more, a paper presenting these results is in progress. It should appear on astro-ph in the next few months! You can also check out other recent SPLASH papers, discussing the extended surface brightness profile of M31 (Gilbert et al. 2012), the properties of the inner regions of M31′s stellar halo (Dorman et al. 2012 and Dorman et al. 2013), and our spectroscopic survey of M31′s dwarf galaxies (Tollerud et al. 2012).

The Hubble and Spitzer Space Telescopes have observed galaxies over 95% of the way back to the Big Bang. The most distant are observed as they were more than 13 billion years ago. These are the building blocks of galaxies like our own Milky Way, which itself dates back to about 13.2 billion years ago.

Two infrared imaging strategies have yielded the most distant galaxy candidates: 1) deep integrations on relatively blank patches of sky such as the Ultra Deep Field (UDF) [1, 2]; and 2) using galaxy clusters as gravitational lenses to magnify the distant universe [3, 4].

Now, for the first time, these two strategies are being combined in the Frontier Fields program. Hubble and Spitzer are obtaining optical and infrared images nearly as deep as the UDF of six lensing galaxy clusters and six nearby “blank” fields over a 3-year period.

The 6 deep blank fields will mitigate the uncertainties previously associated with having only a single UDF (cosmic variance). And factoring in the lensing magnifications, the 6 Hubble cluster images are revealing the faintest sources ever observed (intrinsically nJy, or AB mag > 31).

Figure 1: The second pair of Frontier Fields observed by Hubble: MACSJ0416.1-2403 (left: ACS + WFC3/IR) and a blank field 6′ away (right: WFC3/IR). Each image is ~50″ x 70″, roughly 1/5 the WFC3/IR FOV. The raw HST images have no proprietary period, and STScI processes the images for weekly public releases.

When complete, we estimate these 12 fields may yield ~70 z > 9 candidates (~6 per field), transforming our understanding of the universe’s first 550 million years [5]. To date, only about a dozen candidates are known at these high redshifts [1, 2, 3, 4, 6, 7].

Figure 2: Estimated number counts at z ~ 8 – 12 from the full Frontier Fields program plotted cumulatively as a function of magnitude in the reddest Hubble filter [5]. For these “optimistic” estimates, we extrapolate from a luminosity function evolving with M* consistent with 4 < z < 8 observations. We show predictions for 4 different redshifts both in the field (solid lines) and lensed according to three different publicly available models (dashed lines). The 5-sigma detection limit is F160W AB < 28.7, just twice as bright as the UDF 5-sigma limit F160W AB < 29.45.

Figure 3: Candidate z ~ 7.9 – 8.7 galaxies revealed in deep Hubble WFC3/IR imaging of the blank Frontier Field adjacent to MACSJ0416.1-2403 [5]. Likely due to their high redshifts, these galaxies are detected only faintly if at all in the bluest WFC3/IR filter (1.06 microns). (Alternatively, some may be red early type / dusty galaxies at z ~ 1 – 2.)

Are we witnessing a surprisingly rapid buildup in galaxy numbers during the first 500 million years since the Big Bang? Or are these seemingly surprising results simply a product of cosmic variance and small number statistics? The full Frontier Fields program will mitigate these uncertainties delivering robust population statistics and properties of galaxies in the first 400-550 million years (z ~ 9-11).

For decades, we have sought to use cosmological simulations to better understand our universe as seen by galaxy surveys. Owing to the continually increasing availability of computing power, the latest simulations can directly follow dark matter, stars, and gas in entire galaxy populations. Rapid progress was made toward this goal by adding significant feedback to regulate star formation rates, and by overcoming challenges with numerical methods. Now, hydrodynamical simulations yield increasingly realistic galaxies in large regions of the universe, setting the stage for jointly analyzing the statistics of galaxy morphology in surveys and in theory. Such exercises are needed to measure the observability timescales of galaxy formation events and therefore assign significance to a given observed galaxy state.

Figure 1: Real versus synthetic Hubble Space Telescope (HST) Ultra Deep Field (UDF) from [1]. Left: data from the HST eXtreme Deep Field release [2]. Right: in the same units, intensity, and color scale, a sightline through the Illustris Simulation of galaxy formation, with similar depth and resolution. Current simulations produce galaxies with colors and shapes broadly similar to observed ones.

A number of simulation projects can be analyzed in this manner. For example, the Illustris Simulation [1] calculated galaxy formation over cosmic time in (106 Mpc)3. Illustris applied galaxy physics models consisting of primordial and metal line cooling, star formation, gas recycling, metal enrichment, supermassive black hole (SMBH) growth, and gas heating and ejection by feedback from supernovae and SMBHs. Models were chosen to match the z=0 stellar mass and halo occupation functions, plus the cosmic history of star formation rate (SFR) density, achieving a reasonable mix of spirals and ellipticals.

To make statistical tests regarding galaxy morphology, we process the simulation data into ideal synthetic images, convolve these with telescope point-spread functions, re-bin to CCD pixel scales, and add sky noise. Recent such simulations of thousands of galaxies achieve a resolution element size ~1 kiloparsec, and therefore they are well-matched to imaging by the Sloan Digital Sky Survey at z > 0.05 and Hubble Space Telescope (HST) at z > 1. Fig. 2 shows a sample in mass and redshift as if the galaxies were observed in deep HST images. We are creating synthetic data for up to ~10,000 simulated galaxy histories, at each epoch measuring quantitative automated morphologies from several directions in many filters. With these measurements, we have confirmed that [1] produces a roughly realistic distribution of quantitative galaxy structures at z~0, shown in Fig. 3. Where it is not realistic in the nearby or distant universe, we expect to learn more about the limitations of current large-scale galaxy formation models. Regardless, we are able to extract information about the observability of mergers and other events, which cannot be measured without simulations of this type.

Figure 2: Synthetic HST data of individual galaxies [3], in ACS/F606W, WFC3/F125W and F160W filters with sky noise comparable to the Ultra Deep Field. From left to right in each panel, galaxy models at z=5, 4, 3, 2, 1, and 0. From top to bottom, galaxy models with stellar mass M = 109 to 1011.5 solar masses.

Figure 3: Low-redshift comparison between galaxy morphology from the Illustris Project [1, right] and a volume-limited sample of galaxies [4, left]. This shows current simulations can span the observed space of galaxy structure with roughly the correct dependence on SFR as measured by optical colors.

I have also been studying very high-resolution zoom-in hydrodynamical simulations [5]. These allow accurate modeling of the rest-frame optical and ultraviolet flux via dust radiative transfer [6] to create mock HST images [7], study the effects of dust, and trace galaxy growth and interactions with high precision in both space in time. From many individual galaxy simulations, we are quantifying the distribution of galaxy evolutionary paths and calibrating estimators of the galaxy merger rate at z > 1. We are using multiple simulation sets with different assumptions to verify our estimates of observability timescales.

Galaxy formation simulated at any scale continues to have significant uncertainties, including the treatment of star formation, chemical enrichment, SMBH accretion, and the creation and evolution of outflows. Assumptions made to treat these processes today may be unphysical, leading to low-mass galaxies that are too old at z=0, and SMBH feedback that is too efficient at removing gas from the centers of halos, among other issues. Even so, recent progress in the field allows us to construct statistically relevant experiments with which to improve our understanding of galaxy physics and robustly measure important events during their formation.

Dust and metals play a central role in radiation transport, chemistry, heating and cooling in galaxies, thus influencing their chemical evolution, the star formation rate, and shaping the different phases of the interstellar medium (ISM). For decades, astronomers have been studying the processes that shape the evolution of dust and the cycles of metals between the different ISM phases, from hot, ionized, diffuse, to cold, dense, and molecular. These processes are fairly well constrained in the solar neighborhood. For instance, depletion studies in the Milky Way show that gas-phase metals accrete onto dust grains in atomic or molecular clouds, leading to a decrease by a factor 2-3 in the gas-to-dust ratio between the diffuse and dense phases [1]. However, the influence of environmental factors like metallicity and energetics (radiation field, shocks) on the evolution of dust between ISM phases is not understood. A particular consequence of the lack of constraints on the gas-to-dust ratio and its variations with density in low-metallicity galaxies is our inability to estimate accurate molecular gas masses.

We have been investigating the lifecycle of dust and metals in the Magellanic Clouds, which are the two closest low-metallicity galaxies located 50 kpc and 62 kpc away respectively, and which have metallicities Z=0.5 Zo and Z = 0.2 Zo respectively. We have been using FIR emission from dust grains seen in Herschel PACS and SPIRE [2, 3] at 15 pc resolution to trace the solid-phase, and H I 21 cm and CO 1-0 rotational emission to estimate gas surface densities at similar resolution (Figure 1).

Figure 1:Dust surface density maps in the LMC (top) and SMC (bottom) estimated from FIR emission seen in Herschel PACS (100 and 160 mm) and SPIRE (250, 350, 500 mm). The H I surface density, traced by its 21 cm emission seen in ATCA+Parkes observations, is indicated by the black contours (10-60 M¤ pc-2 in steps of 10 M¤ pc-2). The molecular gas surface density is traced by its CO rotational emission seen in the MAGMA survey, as shown by the white contours representing the 1.2 K km/s of the CO integrated intensity.

The shape of the FIR SED suggests that the dust composition in the Magellanic Clouds is different from that in the Milky Way, dust grains being richer in amorphous carbon [4]. Additionally, the gas-to-dust ratio in the Magellanic Clouds does not scale linearly with metallicity, with diffuse phase gas-to-dust ratio values of 370 in the LMC and 1300 in the SMC, or 2.5 and 9 times the Milky Way value respectively [5, Figure 2). If the gas-to-dust ratio scaled linearly with metallicity, we would expect values of 300 and 750 in the LMC and SMC respectively. This non-linear relation between dust abundance and metallicity, which is also seen in dwarf galaxies [6], could indicate that a smaller fraction of metals is locked up in dust grains in the diffuse phase of low-metallicity galaxies compared to the Milky Way, and that dust grains are destroyed more efficiently in those galaxies. Two arguments concur with this conclusion. First, the filling factor of the dense phase in low-metallicity galaxies is lower than in the Milky Way. Since dust grains are predominantly destroyed by supernova shocks propagating in the diffuse ISM, one would expect a higher dust destruction rate in those galaxies. Secondly, depletion patterns, although at a preliminary stage, seem to confirm the low dust-to-metal ratio in the LMC and SMC.

Our study of the relation between dust and gas across ISM phases also suggests that the gas-to-dust ratio decreases by a factor 2-3 with increasing density, from the diffuse to the dense phase (Figure 3). In principle, this could be evidence of accretion of gas-phase metals onto dust grains in dense molecular clouds. However, although emission-based tracers of the dust and gas phases in external galaxies are valuable to provide context and map the different components of the ISM, they are limited by large (factors 2-3) systematic uncertainties, and degeneracies. First, one has to assume a FIR dust grain emissivity and a model SED to convert the observed FIR fluxes into a dust surface density. However, the emissivity of dust grains depends on their composition and size, and is not constrained to better than a factor of 2 in the Magellanic Clouds. As a result, a gas-to-dust ratio decrease from the diffuse to the dense phase caused by accretion of gas-phase metals on dust grains in the dense ISM would be degenerate with an increase in dust emissivity due to coagulation in the same density range [5]. Dust abundance variations between ISM phases are also degenerate with the presence of CO-dark molecular gas in the translucent envelopes of molecular clouds, where H2 self-shields and can exist, but CO does not and is photo-dissociated. Due to these degeneracies, we still do not know whether the apparent variations in the gas-to-dust ratio is due to CO-dark H2, dust coagulation, or accretion of gas-phase metals onto dust grains in the dense phase, or a combination of those effects. We are in the process of using theoretical models for each of these processes to estimate how big of an effect they can reasonably contribute to the observed, apparent variation in the gas-to-dust ratio.

Figure 2: Relation between dust and atomic gas surface densities in the LMC (top) and SMC (bottom). The grey-scale corresponds to the density of pixels. The red circles show the binned average relation in the diffuse atomic phase, while the blue circles correspond to the molecular phase. The transition between atomic and molecular phases is indicated by the vertical blue dashed line. In the diffuse phase, the slope of the dust-atomic gas relation corresponds to the gas-to-dust ratio, which has a value of 370 in the LMC and 1270 in the SMC.

Figure 3: Gas-to-dust ratio versus dust surface density in the LMC (top) and SMC (bottom). The different ISM phases, atomic, translucent, molecular, are indicated by red, blue, and green colors respectively. The gas-to-dust ratio appears to decrease by a factor 2-3 across ISM phases. This could be evidence for dust growth in the dense ISM, via accretion of gas-phase metals onto dust grains, but could also be a result of dust emissivity variations due to dust grain coagulation, or of an underestimate of the gas surface density in the translucent and dense phases, where we know CO does not track molecular gas accurately in low-metallicity galaxies due to the reduced dust-shielding and increased photo-dissociation.

As all heavy elements are produced in stars and stellar deaths, the eventual fates of metals are unique tracers of the large scale gas flows driving galaxy evolution. Some metals will remain in the ISM, some will get trapped in stars during subsequent episodes of star formation, and some will be blown out of the galaxy via large-scale galactic winds. I have recently conducted an inventory of metals in and around star forming galaxies at z~0 in order to place constraints on the histories of gas flows into, within, and out of galaxies (Peeples et al. 2014).

Figure 1 shows our main result, the relative distribution of metals in galactic and circumgalactic components relative to the total amount of metals galaxies have produced, as a function of galaxy stellar mass. “100%” on this diagram denotes the total mass of metals a galaxy has produced by Type II supernovae, Type Ia supernovae, and AGB stars throughout its lifetime.

Figure 1: Cumulative fraction of metals in interstellar gas (blue), stars (red), interstellar dust (orange), the highly ionized circumgalactic medium (CGM; green), the low-ionization CGM (purple), circumgalactic dust (brown), and the hot X-ray traced CGM (yellow) of star forming galaxies. The points correspond to the median stellar mass of the COS-Halos galaxies. 100% corresponds to the total mass of metals a typical star forming galaxy of a given stellar mass has produced in its lifetime. Adapted from Peeples et al. (2014).

With regards to the metals that remain in galaxies, there are a few surprising results this analysis has uncovered. The relative distribution of metals within galaxies depends on the galaxy stellar mass: massive galaxies have most of their retained metals in stars, while low mass galaxies have the bulk of their retained metals in the interstellar medium. Remarkably, star forming dwarf galaxies have more metals in interstellar dust than in stars; models of the chemical evolution of these galaxies cannot ignore this relatively massive component. Most strikingly, by combining the metals in stars and the ISM gas and dust, we find that star forming galaxies have retained a nearly constant 20-25% of the metals they have produced. That this fraction is so constant is extremely counter-intuitive because our current understanding of galaxy evolution has as a central facet the idea that low mass galaxies are more efficient at driving galactic winds, owing to their relatively shallow potential wells.

An important implication of the fact that galaxies have retained only ~20% of their metals is that, because this fraction is so low, most of the metals galaxies have produced must no longer be in the galaxies, but instead have been expelled into their surroundings. With the installation of the Cosmic Origins Spectrograph (COS) on HST, we can finally systematically characterize galaxies’ gaseous halos’ baryons and metals. With 129 orbits, the COS-Halos program (Tumlinson et al., 2013) has measured the mass of metals and baryons in the circumgalactic medium (CGM) of galaxies with stellar mass ~1010 Msun out to impact parameters of 150 kpc (Peeples et al., 2014; Werk et al., 2014). We find that the CGM is multi-phase: there is a warm, diffuse, highly-ionized phase (traced by the highly ionized oxygen ion, OVI) and a colder, denser low-ionization phase (traced by lower ionization species such as Si II, Mg II, and C II). The relative masses we measure for these phases are shown as the purple and green wedges in the Figure; the points denote the median stellar mass of the COS-Halos galaxies. (The wedge-like shape owes to the fact that we do not detect a difference in the CGM mass within 150kpc that depends on the galaxy stellar mass.) We find that the CGM out to 150kpc has a mass of metals comparable to the mass remaining in the interstellar medium.

The COS-Halos team is now working on observationally extending this metal inventory by characterizing the CGM beyond 150kpc, and, with the COS-Dwarfs survey, of lower mass galaxies within 150kpc. From a theoretical standpoint, I am using analytic models and hydrodynamic simulations to try to reconcile how galaxies can have outflow efficiencies that strongly depend on the depth of their potential wells while simultaneously retaining a constant fraction of the metals they produce and maintaining a CGM structure that is consistent with our observations.

This Month’s Featured Author

Dr. Brian Williams received his B.S. from Florida State University in 2004 and his Ph.D. from North Carolina State University in 2010. He was a NASA Postdoctoral Fellow at NASA Goddard Space Flight Center for three years, after which he worked as a research scientist at NASA GSFC with Universities Space Research Association. He arrived at STScI in February of 2017, and is currently a Support Scientist in the Science Mission Office. His research interests include supernovae and supernova remnants, shock physics and particle acceleration, and dust in the interstellar medium.